The Stoichiometry, kinetics, and mechanism of oxidation of L-cysteine by hexacyanoferrate (III) in acidic media at constant ionic strength have been studied spectrophotometrically. A1:1 L-cysteine:hexacyanoferrate (III) has been established with 1..-cysteine as the only product . The reaction is zero order in hexacyanoferrate (III), first order in L-cysteine and inverse first in hydrogen ion concentration. The activation parameters were evaluated in the temperature range 15-40°C (Ea.# = 28.2 kJ/mol. ΔS# = -192.4 J/mol. deg). The oxidation was inhibited in the presence of added hexacyanoferrate (II). The effects of other added salts and composition of the solvent have been rationalized. Two mechanisms are offered to explain the kinetic data.